In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a liquid polishing slurry consisting mainly of colloidal silica suspended in deionixed water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
Wastewater from the liquid polishing slurry used in the chemical mechanical polishing process must be properly treated for the removal of copper and other chemicals, as well as slurry particles, from the slurry prior to disposal. A typical conventional wastewater treatment system 10 is shown schematically in FIG. 1. The wastewater treatment system 10 includes a wastewater collection tank 12 which receives wastewater from both a CMP apparatus (not shown) and a Cu-CMP apparatus (not shown). The wastewater is distributed from the collection tank 12, into a holding tank 14 by operation of a set of pumps 16.
A second set of pumps 18 pumps the wastewater from the holding tank 14 into a reaction tank 20. Sodium hydroxide (NaOH) base and sulfuric acid (H2SO4) may be distributed into the reaction tank 20 in various proportions to achieve a desired pH of the wastewater in the reaction tank 20. Selected quantities of FSC-835 polymer are further distributed into the reaction tank 20, where the FSC-835 polymer is rapidly mixed with the wastewater to bind or coagulate with the slurry chemicals in the wastewater. A reaction tank outlet line 22 distributes the wastewater, with polymer-bound precipitates, from the reaction tank 20 to one or multiple clarifiers 24. EA-630 polymer is introduced into the reaction tank outlet line 22 to bind remaining slurry chemicals in the wastewater before the wastewater enters the clarifiers 24. In the clarifiers 24, the polymer-bound precipitate particles separate out from the wastewater, which is then distributed to an effluent collection tank 26 through a clarifier outlet line 28. In the effluent collection tank 26, further adjustments may be made to the pH of the wastewater. Finally, the wastewater effluent is distributed from the effluent collection tank 26, through an effluent line 30 and into a reused city water (RCW) tank 34, by operation of a set of pumps 32. The wastewater effluent in the RCW tank 34 may be used for inferior water usage, such as, for example, in local scrubber cleaning applications.
While the FSC-835 and EA-630 polymers have been shown to adequately coagulate and precipitate out of solution chemicals in wastewater from slurry used in most chemical mechanical polishing applications, because the wastewater collection tank 12 receives wastewater from both CMP and Cu-CMP processes, excessive quantities of the polymers are necessary to adequately precipitate the high quantities of slurry chemicals in the wastewater. This contributes to unnecessary expense in the wastewater treatment process. Accordingly, a new and improved, more efficient system and process is needed for precipitating slurry chemicals in CMP wastewater for the proper treatment and disposal of the wastewater.
An object of the present invention is to provide a new and improved system for treating wastewater.
Another object of the present invention is to provide a new and improved system which is suitable for the treatment of CMP and Cu-CMP wastewater.
Still another object of the present invention is to provide a new and improved wastewater treatment system which is efficient and economical.
Yet another object of the present invention is to provide a new and improved wastewater treatment system which facilitates the efficient use of coagulant polymers in the coagulation and precipitation of particles in wastewater.
A still further object of the present invention is to provide a new and improved wastewater treatment system which combines backside grinding wastewater and/or backwash treatment wastewater with CMP wastewater to enhance coagulation and precipitation of particles in CMP wastewater.
Yet another object of the present invention is to provide a new and improved wastewater treatment system which is applicable to the treatment of wastewater in a variety of industrial applications and is not limited to use in treating CMP wastewater.
Another object of the present invention is to provide a new and improved method for the treatment of wastewater.